This invention is directed at an improved fluorosulfuric acid alkylation process. More specifically, the present process is directed at a method for reclaiming products present in a residual bottoms stream from a spent fluorosulfuric acid alkylation regeneration process.
As the use of unleaded gasoline has increased and the quality of crude hydrocarbon feedstocks has declined, it has become increasingly important to utilize processes which improve the octane rating of motor fuels. Alkylation processes, directed at the acid catalyzed reaction of an olefin and a paraffin, are widely used to improve the octane rating of motor fuels. Frequently, sulfuric acid and hydrofluoric acid have been used as the acid catalysts.
Fluorosulfuric acid catalysts have been found to produce alkylates having about 2-4 higher octane numbers than sulfuric or hydrofluoric acid catalysts. U.S. Pat. Nos. 2,313,103; 2,344,469; 4,041,100; and 4,065,516 and U.K. Pat. No. 537,589, the disclosures of which are incorporated herein by reference, describe alkylation processes utilizing fluorosulfuric acid catalysts.
Accordingly, it may be desirable to convert sulfuric or hydrofluoric acid alkylation facilities to the use of fluorosulfuric acid. The major equipment for the fluorosulfuric acid alkylation process comprises a reactor-settler combination, similar to that used for the other two acid catalysts. A two-phase mixture of acid catalyst and hydrocarbons is circulated from the reactor to the settler, where the two phases are separated. The hydrocarbon phase is removed from the settler for distillation, while the acid phase from the settler is divided into two streams, the larger of which is returned to the reactor for further catalysis and the smaller of which is sent for regeneration. The need for regeneration of alkylation catalysts is caused by the accumulation of polymeric hydrocarbons and water in the acid until concentrations are reached which are detrimental to alkylate octane.
In the regeneration of fluorosulfuric acid alkylation catalyst, the major portion of the fluorides can be recovered by stripping the spent fluid. However, one of the major problems associated with the use of fluorosulfuric acid catalyst is the disposal of the residual bottoms which remain after the major portion of the fluorides present in the spent acid have been recovered in the stripping step. The residuum cannot be reprocessed or reclaimed in conventional sulfuric acid recovery facilities because of the corrosion and catalyst poisoning atttributable to the presence of small amounts of residual fluorides.
U.S. Pat. No. 4,033,899 discloses a method for the recovery of a fluorosulfonic acid-sulfonic acid catalyst. In this process the spent liquid catalyst is contacted with a bed of silica alumina catalyst at an elevated temperature for an extended period of time, typically 2-6 hours, to remove fluoride compounds. The effluent having a significantly reduced fluoride content is combusted to convert the sulfuric acid to sulfur dioxide. The sulfur dioxide may be passed over an oxidation catalyst such as vanadium pentoxide in the presence of air to convert the sulfur dioxide to sulfur trioxide which may be absorbed in fresh sulfuric acid solution for reuse. In this process, fluorine may react with the silica to form silicon tetrafluoride, which may be hydrolyzed into hydrogen fluoride in the atmosphere.
U.S. Pat. No. 3,976,759 discloses a process for recovering an alkylation catalyst comprising a major amount of sulfuric acid and a minor amount of fluorosulfonic acid. The catalyst is hydrolyzed and distilled to remove most of the fluorides. The hydrogen fluoride in the distillate is converted to fluorosulfonic acid for reuse. The bottoms comprising substantially all the sulfuric acid and less than 10 ppm of hydrofluoric acid is combusted to SO.sub.2. The SO.sub.2 subsequently is oxidized to SO.sub.3 and absorbed in fresh sulfuric acid for reuse. However, the presence of even about 10 ppm of hydrofluoric acid in the bottoms stream may cause corrosion of the SO.sub.2 handling facilities and may poison the catalyst utilized to oxidize SO.sub.2 to SO.sub.3.
Bartkiewicz and Robinson, in "Rapid Method for the Determination of Fluorine in Liquids" Anal. Chem. Acta, 22, (1960) pp 427-431 disclose that fluoride containing gases can be recovered in an aqueous scrubbing solution.
Kohl and Riesenfeld report in Gas Purification, Third Edition, Gulf Publishing Co., Houston, Tex. (1979) several methods for removing sulfur oxides from gas streams. At pages 410-421 and 679-686, methods for the recovery of sulfur oxides from gas streams utilizing the Claus process alone or in combination with other processes is reported.
U.S. Pat. Nos. 3,295,318; 4,071,576; 4,073,821; 4,096,197; 4,096,198; and 4,096,199 disclose methods for recovery and/or regenerating fluorosulfuric acid alkylation catalysts.
Karchmer, J. H. (ed), in "The Analytical Chemistry of Sulfur and Its Compounds Part I", Wiley Interscience (1970) discusses the chemistry of aqueous solutions of sulfur oxides.
Accordingly, it is desirable to provide a process in which the residual bottoms from a fluorosulfuric acid catalyzed alkylation process are reprocessed.
It is also desirable to provide a process in which the treatment of the residual bottoms is incorporated into a conventional sulfuric acid alkylation facility without the addition of an excessive amount of equipment.
It also is desirable to provide a process in which the sulfur present in the bottoms stream is recovered.
It is further desirable to provide a process in which the fluorides ultimately discharged are rendered relatively inert.
It also is desirable to provide a process in which corrosion resulting from bottoms processing is minimized.
It also is desirable to provide a process in which fluoride discharge to the atmosphere is minimized.
The present invention is directed at a method for treating residual regenerator bottoms comprising acid-soluble oil and a minor amount of fluorosulfuric acid. The method is directed at sequentially combusting the residual bottoms, scrubbing the resulting combustion products to remove fluorides, and treating the sulfur oxides. In a preferred embodiment the sulfur oxides are passed to a conventional sulfur treating facility, such as a Claus plant. The scrubbing solution used to absorb the fluorides preferably is contacted with lime to precipitate the fluorides present.